In recent years, it is a common practice for a parallel computer system to improve its performance by increasing the number of nodes rather than increasing the size of each node.
In a parallel computer system in general, computation is executed while transmitting and receiving data between nodes by a network line (interconnect) which connects the nodes. As a method of connecting a network line in this case, while complete connection of directly connecting one arbitrary node and all the other nodes is optimum in terms of communication efficiency, when the number of nodes is increased, this method is not practical because of difficulty in packaging and costs.
Adopted in many cases therefore is a method, with adjacent nodes directly connected with each other, of communicating with nodes provided farther through a plurality of nodes until reaching a target node from an adjacent node. One of such methods is a connection method based on a two-dimensional torus or a three-dimension torus.
In a case of such a connection method, however, communication with an adjacent node directly connected by a network line and communication with a node (far-away node in terms of a network) which should be communicated with through a plurality of nodes differ from each other in time required for communication.
More specifically, it is commonly known that in a connection method based on a two-dimensional torus or a three-dimensional torus, a TAT (Turn Around Time) of a job executed on a parallel computer system might be longer than that in a case of complete connection by which communication with all the nodes is completed in the same time period.
This might be a big problem, taking the fact that the number of nodes will be further increased. Therefore, in a case of executing a job which uses a plurality of nodes on a parallel computer system, as close a node as possible in terms of a network should be selected.
As related art for selecting as close a node as possible in terms of network in a parallel computer system, recited in Patent Literature 1, for example, are determining an unallocated subset from a plurality of HPC (High Performance Computer) nodes and a case where for minimizing a distance between nodes, the best fit is a cube or a sphere depending on a kind of job.
Patent Literature 1, however, fails to recite an actual method of determining an “unallocated subset” in a case where other plurality of jobs being executed already exist, and a node in use and a free node exist together in a parallel computer system.    Patent Literature 1: Japanese Patent Laying-Open No. 2005-310139.
As one example of commonly possible methods is first selecting an arbitrary one node from a set of free nodes and in order to check how many free nodes exist among its surrounding node, scanning the surrounding nodes. When few free nodes exist in the surroundings of the node, another free node should be again selected, which operation should be repeated until an appropriate free region is found. In this case, when the above-described operation is repeated until an appropriate free region is found, labor of O(n^2) (n represents the number of nodes, which is a positive integer) might be cost in some cases.
From the foregoing, the problem to be solved is providing a method of efficiently finding a free region (a region where free nodes exist in abundance) from a two-dimensional torus or a three-dimensional torus space particularly when a plurality of jobs using a plurality of nodes exist.